The proposed research is designed to test several hypotheses which concern the regulation and significance of hepatic cGMP. In contrast to cAMP which is produced by plasma membrane bound adenylate cyclase in response to specific stimuli, the control of cGMP levels is thought to occur at specific intracellular loci. The first objective is to detemine the localization of cGMP after hormonal stimulation and liver regeneration. Immunohistochemical technique and subcellular fractionation will be used to study the compartmentalization of cGMP. The guanylate cyclase also seems to have a characteristic subcellular location with at least three forms present in the liver. These forms are soluble, particulate and nuclear. The guanylate cyclase activity assay will be used to further characterize the subcellular localization of guanylate cyclase, the in vitro regulation of guanylate cyclase activity by ions and nucleotides, the positive and inhibitory effects which cAMP has on guanylate cyclase activity and the alterations that occur in guanylate cyclase distribution following partial hepatectomy. The last objective will be to determine the intranuclear role of cGMP during the rapid but orderly growth of the liver which occurs following partial hepatectomy. The significance section of this grant emphasizes the importance of this work to a basic understanding of the hormonal (insulin, glucagon, etc.) regulation of hepatic processes and their role in liver disease and diabetes.